Recently, regulations concerning automotive exhaust gas have been tightened, and accordingly there has been an increase in the number of cases where a metal honeycomb made of stainless steel foil serving as a catalyst carrier for an automotive exhaust gas purifying facility is installed in an automobile. A metal honeycomb is capable of realizing a larger aperture ratio and has higher thermal shock resistance and higher vibration resistance than a ceramic honeycomb carrier. Therefore, the proportion of cases where a metal honeycomb is employed has increased. In particular, when exhaust gas purifying facility is installed in a large automobile such as a truck, the shape of the carrier accordingly becomes large and therefore a metal honeycomb is commonly employed due to its flexibility of shape.
A metal honeycomb has a honeycomb structure formed by, for example, stacking a flat stainless steel foil (flat foil) and a stainless steel foil that has been worked into a corrugated shape (corrugated foil) alternately. A catalytic material is applied to the surfaces of the stacked stainless steel foils, and the resulting metal honeycomb is used for an automotive exhaust gas purifying facility. FIG. 1 shows an example of a metal honeycomb; a metal honeycomb 4 is prepared by stacking a flat foil 1 and a corrugated foil 2, winding the resulting product into a roll shape, and fixing the periphery of the wound product in place with an external cylinder 3.
Addition of more than about 3 mass % of Al to stainless steel causes an Al2O3 oxide layer to be formed on the surface of the stainless steel, which markedly improves oxidation resistance. Therefore, high-Al content ferritic stainless steel foil, typified by 20 mass % Cr-5 mass % Al stainless steel foil or 18 mass % Cr-3 mass % Al stainless steel foil, has been mainly used as stainless steel foil used for forming metal honeycombs.
In a gasoline-powered automobile, the temperature inside its automotive exhaust gas purifying facility is increased due to the temperature of exhaust gas and a catalytic reaction and may reach a high temperature of 1000° C. or more. Thus, high-Al content ferritic stainless steel foil having considerably high oxidation resistance at high temperatures is used as a catalyst carrier. A catalyst carrier requires, in addition to oxidation resistance, resistance to shape change even when repeatedly exposed to a high temperature, resistance to peeling of an oxide layer that occurs during cooling of the catalyst carrier, and high adhesion to wash coat. High-Al content ferritic stainless steel foil is good in terms of these properties and thus has been widely used.
On the other hand, the temperature of exhaust gas of a diesel-powered automobile does not raise as high as the temperature of exhaust gas of a gasoline-powered automobile, and the temperature reached is generally about 800° C. The highest temperature reached by exhaust gas of a vehicle other than automobiles, such as agricultural machinery or building machinery, is even lower. Therefore, when the above-described 20 mass % Cr-5 mass % Al stainless steel foil or 18 mass % Cr-3 mass % Al stainless steel foil is applied to a vehicle that discharges exhaust gas having a low temperature, such as a diesel-powered automobile, in most cases, high oxidation resistance becomes redundant. In addition, while the above-described high-Al content ferritic stainless steel foil has high oxidation resistance, the manufacturability of the foil is poor and consequently its production cost is increased. Generally, addition of Al to ferritic stainless steel tends to significantly reduce the toughness of the ferritic stainless steel. This often results in rupturing of a steel sheet during cooling of a cast slab, during expansion of a hot-rolled coil, or during cold rolling, which causes equipment trouble and a reduction in yield.
In order to address the above-described problems, stainless steel foil whose manufacturability is improved by reducing the Al content in the stainless steel foil as much as possible has been proposed. For example, Patent Literature 1 discloses a metal honeycomb made of stainless steel foil produced by limiting the Al content to be within the range from a level of impurities to 0.8 mass % and generating a Cr oxide layer at a high temperature but not an Al oxide layer in order to improve diffusion bondability for forming a carrier.
Patent Literature 2 discloses a metal honeycomb made of stainless steel foil produced by limiting the Al content in the stainless steel foil to be within the range from a level of impurities to 0.8 mass % and setting the Mo content to 0.3 mass % to 3 mass % in order to improve oxidation resistance, diffusion bondability, and resistance to corrosion by sulfuric acid.
Patent Literature 3 discloses stainless steel produced using 18 mass % Cr-3 mass % Al steel as a base material by reducing the Al content in the steel to be less than 1.5 mass % to 2.5 mass % and limiting the crystal grain size in order to realize both good workability and high oxidation resistance.